Over the last decade or so, semiconductors have become an essential tool in building consumer products. During this time, ICs have needed to shrink in physical size so that the consumer products can be smaller. ICs have also needed to increase in performance. For example memory chips have quadrupled in memory size about every two to five years. Now, ICs must use less power to operate in the new battery powered portable consumer products. Thus, the IC has moved from requiring 12 volts to 3.3 volts and even less. Consumers expect smaller products that run longer between recharges, while having a vast array of options.
For the purpose of providing background material which may in some respects illustrate the state of the art, consider the circuit of FIG. 5 which shows an example of a V.sub.T referenced source, also called a bootstrap reference. M3 and M4 cause the currents I.sub.1 and I.sub.2 to be equal. I.sub.1 flows through M1 creating a voltage V.sub.GS1. I.sub.2 flows through R creating a voltage I.sub.2 R. Because these two voltages are connected together, an equilibrium point is established. The equation describing this equilibrium point Q is given as ##EQU1## This equation can be solved iteratively for I.sub.1 =I.sub.2 =I.sub.Q or alternately, one can assume that V.sub.GS1 is approximately equal to V.sub.T1 so that ##EQU2## Since I.sub.1 or I.sub.2 does not change as a function of V.sub.DD, the sensitivity of I.sub.Q to V.sub.DD is essentially zero. A voltage reference can be achieved by mirroring I.sub.2 (=I.sub.Q) through M5 or M6 and using a resistor.
Unfortunately there are two possible equilibrium points. One is at Q and the other is at zero. In order to prevent the circuit from choosing the wrong equilibrium point, a start-up circuit is necessary. The dotted circuit functions as a start-up circuit. If the circuit is at the undesired equilibrium point, then I.sub.1 and I.sub.2 are zero. However, M7 will provide a current in M1 that will cause the circuit to move to the equilibrium point at Q. As the circuit approaches the point Q, the source voltage of M7 increases causing the current through M7 to decrease. At Q the current through M1 is essentially the current through M3.